Method for inhibiting stains on aluminum product surfaces

ABSTRACT

A method for inhibiting the formation of stains, especailly water stains, on the exterior surface of aluminum alloy products. The method entails contacting the exterior surfaces of these products, particularly sheet or plate products, extrusions and/or forgings made from 5000 or 6000 Series aluminum alloys, with an organophosphonic or organophosphinic acid-derived material. Preferably, liquid forms of this material are added to an alcohol or water-based carrier solution, then sprayed, dipped, painted or rolled onto the surfaces of flat sheet or plate products to enhance their brightness. Other more complex shapes may be dipped into material baths.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of application Ser.No. 09/335,676, filed on Jun. 18, 1999, which application claims thebenefit of U.S. Provisional Application Ser. No. 60/090,017, filed onJun. 19, 1998, the disclosure of which is fully incorporated byreference.

FIELD OF THE INVENTION

[0002] 1. Background of the Invention

[0003] Water stains do not generally present problems for the structuralproperties and/or corrosion performance of an aluminum product. Aluminumsurface discolorations that accompany water staining may discomfort somecustomers who are unfamiliar with the surface and corrosion propertiesof aluminum. Customers already aware of the properties of cold rolledsteel may mistakenly believe that water stains on aluminum are the onsetof “rusting”, similar to that found on steel. For bright aluminumproducts, such as buffed trailer plate, rail cars, toolboxes, runningboards, and tread plate on fire trucks, stain inhibition would preservethe buffed famish and enhance customer satisfaction. A simple, low-costsolution to inhibit water stain on aluminum could result in a higherdegree of customer confidence in replacing steel with aluminum for theirproducts. In addition, aesthetics of these products is important to theend customer. Water stains are aesthetically unattractive and theirelimination or reduction would be valuable to the owner whether it be analuminum trailer, rail car, toolbox or other aluminum product.

[0004] 2. Description of the Relevant Art

[0005] Numerous uses for organophosphonic acids in conjunction withaluminum are known. These include U.S. Pat. Nos. 4,957,890, 5,032,237,5,059,258, 5,103,550, 5,124,022, 5,124,289, 5,126,210, 5,132,181,5,238,715, 5,277,788 and 5,463,804. None of these, however, mentionorganophosphonic acids for the inhibition of stains, especially waterstains, on aluminum surfaces. Most of the aforementioned patentsdescribe aluminum surface pretreatments that enhance the durability oforganic coatings or adhesively bonded joints. They do not describe theuse of organophosphonics without a topcoat.

[0006] Other methods for inhibiting corrosion with respect to aluminumand other metals are disclosed in U.S. Pat. Nos. 3,433,577, 3,672,822and 4,427,448.

BRIEF SUMMARY OF THE INVENTION

[0007] This invention addresses a low cost method for inhibiting waterstaining on 5000 Series, or 5XXX, aluminum alloys, most notably5083-H321 and 5454-H32 aluminum (Aluminum Association designations).Such alloys are used to make rail hopper cars and buffed trailer tanks.Similar surprising and unexpected results have been observed when thismethod was practiced on 6000 Series aluminum alloys, like the 6061-T6alloys used to make various products including vehicle wheels. Accordingto this method, it was determined that spraying a solution consisting ofabout 0.25 wt % octadecylphosphonic acid (or “ODPA”) in an isopropanolsolvent (or other medium) onto these aluminum alloy products, thenallowing the alcohol to evaporate, is effective for inhibiting waterstaining. Similar effects were subsequently observed with a carriercomposition containing octylphosphonic acid (or “OPA”). Suitable liquidcarriers include alcohols, ketones, ethers, aldehydes, alkanes, andother organic solvents with sufficient solubility for theorganophosphonic acids. These organophosphonic acid-derived solutionscan be applied to the metal surface by spraying, dipping, painting, orroll coating. It is also recognized that the stain inhibitor componentcan be delivered to the aluminum surfaces from various compositions usedin the manufacture of aluminum parts, including but not limited to:aqueous suspensions or solutions; metal forming lubricants, and metalcleaning and/or rinsing formulations; a buffing compound or wax thatincorporates the stain inhibitor, metal heat treatment quench waters,and/or post-rinsing polishers/sealants or the like. For certain staininhibitor compounds, it is possible to buff a paste-like stain inhibitordirectly onto the aluminum product surface.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Further features, objectives and advantages of the presentinvention will be made clearer from the following detailed descriptionmade with reference to the drawing in which:

[0009] The FIGURE shows the schematic formation and orientation ofhydrolytically stable Al-O-P bonds of the stain inhibitor,octylphosphonic acid (OPA), as a reaction product with an oxidizedaluminum surface for effecting the stain inhibition observed accordingto this invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0010] Preliminary indications of the effectiveness of this invention,for inhibiting stains, were observed in an accelerated corrosion testthat involved outgassing products of ethylvinylacetate plastic pellets(obtained from Millennium Petrochemicals), high humidity and temperaturecycling. After 12 temperature cycles, no water stains were observed on5000 Series alloy samples initially etched in caustic, then sprayed withan ODPA-containing solution. Water staining was also inhibited for “millfinish” metal sprayed with ODPA; though some spots were interspersedwith unstained surface in the latter case. By contrast, mill finish andetched-only samples were completely covered with water stains. It isbelieved that the difference in performance as a result of pre-etchingwere most likely due to the removal of residual rolling lubricants viaetching. In that manner, the stain inhibiting molecules of thisinvention would be allowed to chemically bond with surface aluminumoxides.

[0011] Chemical reaction of the inhibitor to the surface can also beachieved by changing the means of application or by using a differentsolvent. The surface ODPA inhibits access of water to the aluminum oxideand forms hydrolytically stable bonds with the oxide, thus inhibitingwater staining. ODPA is a commercial compound manufactured and sold byAlbright & Wilson Ltd. Working solution concentrations and surfacecoverages of this invention are relatively low, which results in lowtreatment costs of cents per square foot of Al plate or sheet product.The same would be true for other aluminum product forms, includingcastings, forgings and extrusions.

[0012] Another potential stain inhibitor, octylphosphonic acid (OPA),was evaluated. It showed even better performance results than the ODPAsamplings above. OPA has the following chemical structure:CH₃(CH₂)₇P(O)(OH)₂. It can be applied with a water and surfactantcarrier as effectively as with an isopropanol carrier. OPA is moresoluble than ODPA in isopropanol thus allowing for increased solutionconcentrations. And while OPA is not water soluble, it forms asuspension of solids with water. When in water, no volatile organiccompounds (or VOC's) result from metal contact for stain inhibitors.

[0013] Preliminary humidity test results show that OPA is highlyeffective for inhibiting stains on mill finish or buffed aluminumproducts without cleaning, pickling or pre-etching. After three hours at50° C. (125° F.) and 100% relative humidity, the OPA treated surface wasunstained, whereas “as-buffed”, untreated surfaces were considerablystained.

[0014] It may also be possible to apply certain formulations by themethods of this invention with no carrier solution. For example, one maydirectly buff a more solid form of OPA onto an aluminum product surface.It is also possible, actually even more practical depending on thealuminum surface to be treated, to incorporate the stain inhibitingcompounds of this invention into mill lubricants for providing an insitu type of stain inhibition and eliminating subsequent processingsteps. It may also be possible to similarly add such stain inhibitors tobuffing, sealing and/or polishing compound formulations.

[0015] When the aluminum to be treated is mill finish or “as-buffed”, apreferred carrier/solvent is an alcohol, more preferably 2-propanol orisopropanol. Isopropanol is also beneficial in that its solvent actionis believed to displace residual mill lubricants or buffing compoundsand wet the surface aluminum resulting in the formation of Al-O-P bondswith the oxidized aluminum surface. Isopropanol is also non-toxic. Whenthe aluminum surface has been pre-cleaned or etched, the choice ofsolvent is not as critical. In many instances, water may be used totransport (or apply) such stain inhibitors.

[0016] In addition to forming hydrolytically stable Al-O-P bonds,organophosphonic acids may provide yet another mechanism for staininhibition. For example, when OPA or ODPA reacts on the Al surface, thereaction end product is believed to orient or align so that itshydrocarbon chains extend away from said surface. A schematicrepresentation of the bonding that is believed to take place is shown inthe accompanying FIGURE. The latter surface takes on a “hydrophobic” ornon-wetting quality thereby further inhibiting the conversion of oxidesto hydroxides (or effecting a water stain thereon). Under the latterscenario, longer chained organophosphonic acids become the preferredstain inhibitors for this invention.

[0017] In some embodiments of this invention, a full (and not partial ornon-uniform) haze on the aluminum product surface may form. It ispreferred that such haze be wiped away with a dry cloth to furtherenhance stain inhibition. On a less preferred basis, this haze may beremoved by rinsing the aluminum product's outer surface.

[0018] Certain classes of phosphorus oxo acids, acid esters, and acidsalts are effective to various degrees in preventing water stainsaccording to this invention. Phosphate salts, phosphate esters, andphosphonic acids each impart some stain inhibition. In comparativetests, however, octadecylphosphonic (C-18) acid (ODPA) and severalfluoro-phosphonic acids were not as effective as OPA (C-8) in inhibitingstains. Poly(vinylphosphonic acid), and copolymers thereof, may workeven better than OPA, but it is currently cost prohibitive to use incommercial quantities. Some of the representative stain inhibitors canbe grouped by the following “families”:

[0019] a) acidic aluminum phosphate salts

[0020] b) inorganic phosphorus oxo acids

[0021] c) organophosphonic and organophosphinic acids

[0022] d) phosphate acid esters

[0023] e) organo phosphonic acid polymers and copolymers; and forexample, poly(vinyl phosphonic-co-acrylic acid)

[0024] f) phosphate ester polymers for example, poly(vinyl phosphoricacid).

[0025] This invention can be used to improve the stain inhibition ofnumerous aluminum alloy surfaces, including various sheet or plateproducts, extrusions and forgings, regardless of whether such productshave welded joints or other connections. It is best suited for anyaluminum product that its purchaser, the end user/consumer, would preferthat said product “look good” (i.e. brighter, less stained, etc.)longer! This includes a whole family of building/architectural products,appliances, lighting supplies, and other household cosmetics likevertical blind stock. On a preferred basis, the method of this inventionworks well with 5000 and 6000 Series alloys (Aluminum Associationdesignation). It should also enhance the stain inhibiting performance ofproducts made from other aluminum alloys, including but not limited to1000 and 3000 Series alloys.

[0026] The primary focus of this invention is to inhibit the formationof stains on the exposed outer surfaces of aluminum products. The methodclaimed below preferably accomplishes this objective using anoctylphosphonic acid-based aqueous solution. That solution can beapplied to various product forms in a variety of ways. More importantly,this invention is believed to achieve its desired result by decreasingthe interaction with, or adhesion of, materials to the aluminum surfacesso treated. It is especially desired to inhibit the interaction ofaluminum's outer surface oxide layer (the layer that naturally forms onuncoated or exposed Al product) with waters including rainwater or thelike. It is through such contact with rainwater, etc. that aluminumsheet, plate, forgings and extruded product forms result in a visiblystained, or “dulled” exterior surface—an undesirable surface appearancefor the owners of such otherwise shiny product forms.

[0027] A primary focus of Alcoa's earlier patents, those to McCleary etal (U.S. Pat. No. 5,463,804) and Wefers et al (U.S. Pat. No. 5,103,550),was to IMPROVE the adhesion of coatings to aluminum stock using avinylphosphonic acid-based formulation. That VPA-derived compositionactually served to increase the interaction of adhesives or coatingswith their treated aluminum surfaces, primarily vehicle assemblies forMcCleary and beverage container stock for Wefers. In order to determinewhether the formulations of McCleary and/or Wefers, distinctly differentfrom those of the present invention, have similar stain-inhibitingpotential, a somewhat crude analysis was conducted comparing theoctylphosphonic acid-based solutions of this invention with theVPA-derived varieties of these two prior art references. For thisanalysis, bright coupons of uncoated 5454 aluminum (Aluminum Associationdesignation) were contacted with either: (a) the stain fightercomposition of this invention; or (b) a VPA-pretreatment representativeof the aforementioned prior art. Those coupons were then subjected tohumidity cabinet tests, aimed at simulating accelerated outdoorweathering conditions. The coupons treated with a VPA-based solutionexhibited unacceptable water staining after only 5 hours of exposure. Bycontrast, specimens treated per the present invention remainedstain-free after over 50 hours of humidity cabinet weatheringsimulation. As such, the adhesive-enhancing formulations of McCleary andWefers do not teach stain-inhibiting methods, but rather teach away fromthe presently claimed invention.

EXAMPLES

[0028] First Study—Several sections of buffed trailer tank plate product(made from 5454 aluminum alloy) were sprayed with two comparative staininhibiting compositions:

[0029] Set 1: 0.2 wt % octylphosphonic acid (OPA) in isopropanol; and

[0030] Set 2: 0.2 wt % octadecylphosphonic acid (ODPA) in isopropanol.

[0031] Haze on both sets of sprayed plates was rinsed away with water,then gently buffed with dry cheesecloth. These treated plates, alongwith an “as-buffed” control, were then placed in a humidity cabinet at50° C. (125° F.) with 100% relative humidity for 3 hours. Afterexposure, the plates were removed from the cabinet, dried with a towel,and visually examined for staining.

[0032] No noticeable loss of specularity was observed with either of theabove stain inhibition treatments. All surfaces had the same visualappearance as the “as-buffed” sample. After humidity exposure, however,brownish colored, water stains were evident over a majority of the“as-buffed” and ODPA-treated surfaces. The OPA treated surface did notexhibit any water stains and appeared the same as unexposed specimens.While ODPA specimens did not fare as well as OPA in this particularstudy, as compared with its earlier positive results, differentapplication techniques are believed to have caused its reduced staininhibiting performance here.

[0033] Second Study—Tanker Trial results—A covered hopper trailer, madefrom 5454 aluminum Bulk Transportation Sheet (“BTS”) was treated withvarious applications according to the invention before being exposed toharsh, in-service conditions: from an aggressive environment of salt airdue to seacoast proximity; and harsh winter conditions with numerousroad salt applications. Subsections of this hopper/tanker were treatedas follows: (a) 1 wt. % solution of OPA, in isopropanol, was sprayed onthe first section of tanker, dried to a film, water rinsed and air driedthereafter; (b) the same solution as above was sprayed onto anotheradjoining section of the same hopper/tanker, then dried to a film andwiped to an initial shine using cheesecloth; (c) for this section ofhopper/tanker, the treatment material was 1 wt. % OPA, suspended inwater. After spraying, this water-based solution was allowed to sit onthe product surface for about 10 minutes before being dried and wiped toa shine with cheesecloth. The last comparative section of hopper/tankerwas sprayed with a 5 wt. % solution of OPA, in water, before beingallowed to sit for 10 minutes, then water rinsed and air dried.

[0034] After three months of service along the U.S. East Coast, thishopper/trailer was brought back for inspection. While OPA treatmentswere observed to provide a substantial degree of water stain inhibitionover that 3 month trial period, one of the first conclusions drawn fromthat inspection was that monthly reapplications could ensure a pristine,polished surface on such trailer stock.

[0035] Following a wash with non-etching alkaline cleaner, various sidesand subsections of this hopper/tanker were photographed and closelycompared by visual inspection. From that inspection, it was noted thatthe water-based sections of treated hopper fared better than theiralcohol-based counterparts (in terms of water staining inhibition). Inaddition, wiping to a shine after application of the OPA, as per example(a) above, was most effective, even more than merely applying, rinsingand air drying, the latter treatment resulting in a noticeable, residualhaze at first.

[0036] Third Study—Coil Line Trial—A coil of 5182-H19 aluminum sheet wasroll coated with a 5% aqueous suspension of OPA. Phosphorus surfaceconcentrations were measured on the treated surface using X-rayfluorescence spectroscopy (XRF). From previous bench scale tests, it wasobserved that phosphorus surface levels of about 2 Kcps were sufficientfor inhibiting water staining. Phosphorus surface levels on theaforementioned sheet product were measured at about 10 Kcps, however.

[0037] Fourth Study—Forged Truck Wheels—Forged and polished truck wheelsmade from aluminum alloy 6061-T6 were treated with comparative solutionsof 0.5 and 1 wt. % OPA in isopropanol. For testing, the treated wheelswere placed into a cabinet with condensing humidity set at 100° F. Thewheels were examined every hour for water stains. The tests were stoppedafter 120 hours of humidity exposure. Untreated wheels (as-polished)were substantially stained within 11 hours of humidity exposure. Thewheels that were treated with OPA, then buffed to shine lasted thelongest without substantial water staining. On certain OPA-treatedwheels, only a few small, widely dispersed spots were observed after 120hours of exposure testing, but that level of staining was insignificantcompared to the gross quantities of water staining observed on theuntreated wheels after only 11 hours of humidity exposure.

[0038] Fifth Study—Lighting Sheet—Aluminum alloy 5657-H18, used formaking bright lighting sheet, was treated with a 5 wt. % solution of OPAstain inhibitor in isopropanol. Specular reflectance measurements showedthat after buffing the resultant haze from said sheet surfaces, the OPAtreatment did not reduce reflectivity. Furthermore, such OPA-treatedpanels lasted up to 13 days in condensing humidity at 100° F. withoutstaining, as compared to their untreated sheet equivalents that weresignificantly stained within 24 hours of such humidity exposure.

[0039] Sixth Study—Quench Water Additions—Phosphorus compounds, likethose described above, were added to the quench waters used for makingextruded tubes and rolled sheet from 6061-T6 alloy. In this comparison,the aluminum product forms were heated to about 1000° F. before beingcold water quenched, said quenching solution containing variousphosphorus compounds. Thereafter, these products were allowed to remainin the quench water for 24 hours. By visually examining these aluminumproduct forms, and by further measuring the amount of hydroxides formedthereon using Fourier-transform infrared spectroscopy (FT-IR), it wasdetermined separately that 10 g/L solutions of dibasic ammoniumphosphate —(NH₄)₂HPO₄— and 10 g/L phytic acid best prevented theformation of water stains on these products. They also prevented theformation of bayerite powders on the interior aluminum surfaces of theseextruded tubes.

[0040] Having described the presently preferred embodiments, it is to beunderstood that the invention may be otherwise embodied within the scopeof the appended claims.

What is claimed is:
 1. A method for inhibiting formation of stains on anexposed exterior of an aluminum alloy product, said method comprising:(a) contacting the exposed exterior with a material that is capable offorming a hydrolytically stable Al—O—P bond therewith, said materialselected from the group consisting of: an acidic aluminum phosphatesalt; a phosphorous acid, a hypophosphorous acid; an organophosphonic ororganophosphinic acid; a phosphate acid ester; an organophosphonic acidpolymer or copolymer; and an organophosphate acid ester polymer orcopolymer.
 2. The method of claim 1 wherein the material consistsessentially of an organophosphonic or organophosphinic acid.
 3. Themethod of claim 2 wherein the material is a solid or semi-solid at roomtemperature and applied to the exposed exterior of the aluminum alloyproduct.
 4. The method of claim 2 wherein the material is a liquid atroom temperature and combined with a carrier selected from the groupconsisting of: an alcohol, a ketone, an ether, an aldehyde, an alkane,water and mixtures thereof.
 5. The method of claim 4 wherein thematerial consists essentially of octadecylphosphonic acid and thecarrier includes isopropanol.
 6. The method of claim 4 wherein thematerial consists essentially of octylphosphonic acid and the carrier isselected from the group consisting of isopropanol, water and mixturesthereof.
 7. The method of claim 4 wherein said exposed exterior iscontacted with an aqueous solution consisting essentially of up to about10 wt. % octadecylphosphonic acid, about 5-90 wt % isopropanol, abalance of water and impurities.
 8. The method of claim 4 wherein saidexposed exterior is contacted with an aqueous solution consistingessentially of up to about 10 wt. % octylphosphonic acid, about 5-90 wt% isopropanol, a balance of water and impurities.
 9. The method of claim4 wherein said exposed exterior is contacted with an aqueous solutionconsisting essentially of about 0.5-10 wt. % octylphosphonic acid, abalance of water and impurities.
 10. The method of claim 4 wherein saidcontacting includes: spraying, dipping, painting or rolling material onsaid exposed exterior of said alloy product.
 11. The method of claim 10wherein said material is incorporated into a composition for surfacetreating said alloy product.
 12. The method of claim 11 wherein saidcomposition is selected from the group consisting of: a mill lubricant,a quenching solution, an intermediate rinse, an etching solution, asolvent, a surfactant, a cleaner, a polish, a post-rinse, a sealant andmixtures thereof.
 13. The method of claim 2 wherein said alloy productis selected from the group consisting of sheet product, plate product,an extrusion and a forging.
 14. The method of claim 13 wherein saidsheet or plate product has at least one bright exterior surface and isselected from the group consisting of: trailer plate, rail car skin, atool box, vehicle running board and tread plate.
 15. The method of claim13 wherein said sheet product is reflective lighting sheet.
 16. Themethod of claim 13 wherein said extrusion is selected from the groupconsisting of: a truckbed rail, hydraulic tubing, window framing, tuband shower framing, and a greenhouse structural support.
 17. The methodof claim 13 wherein said alloy product is a vehicle wheel.
 18. Themethod of claim 1 which further includes: (b) rinsing material from theexposed exterior.
 19. The method of claim 18 which further includes: (c)wiping said exposed exterior.
 20. The method of claim 1 which furtherincludes: (b) wiping material from the exposed exterior.
 21. The methodof claim 1 wherein said alloy product is made from a 1000, 3000, 5000 or6000 Series aluminum alloy (Aluminum Association designations).
 22. Themethod of claim 21 wherein said 5000 Series aluminum alloy is selectedfrom the group consisting of: 5083, 5086, 5454, 5657, 5182, and 5456aluminum (Aluminum Association designations).
 23. The method of claim 21wherein said 6000 Series aluminum alloy is selected from the groupconsisting of: 6061, 6111 and 6022 aluminum (Aluminum Associationdesignations).
 24. A method for inhibiting the formation of water stainson an exposed exterior surface of a 5000 or 6000 Series aluminum alloyproduct, said method comprising: (a) contacting the exterior surfaceswith an organophosphonic or organophosphinic acid-derived material. 25.The method of claim 24 wherein said acid-derived material is a solid orsemi-solid at room temperature and applied to said exterior surface. 26.The method of claim 24 wherein said acid-derived material is a liquid atroom temperature and combined with a carrier selected from the groupconsisting of: an alcohol, a ketone, an ether, an aldehyde, an alkane,water and mixtures thereof.
 27. The method of claim 26 wherein saidacid-derived material consists essentially of octadecylphosphonic acidand said carrier includes isopropanol.
 28. The method of claim 26wherein said acid-derived material consists essentially ofoctylphosphonic acid and said carrier is selected from the groupconsisting of: isopropanol, water and mixtures thereof.
 29. The methodof claim 26 wherein said exterior surface is contacted with an aqueoussolution consisting essentially of up to about 10 wt. %octadecylphosphonic acid, about 5-90 wt % isopropanol, the balance waterand impurities.
 30. The method of claim 26 wherein said exterior surfaceis contacted with an aqueous solution consisting essentially of up toabout 10 wt. % octylphosphonic acid, about 5-90 wt % isopropanol, thebalance water and impurities.
 31. The method of claim 26 wherein saidexterior surface is contacted with an aqueous solution consistingessentially of about 0.5-10 wt. % octylphosphonic acid, the balancewater and impurities.
 32. The method of claim 24 wherein said contactingincludes: spraying, dipping, painting or rolling material onto saidexterior surfaces.
 33. The method of claim 24 wherein said acid-derivedmaterial is incorporated into a composition selected from the groupconsisting of: a mill lubricant, a quenching solution, an intermediaterinse, an etching solution, a solvent, a surfactant, a cleaner, apolish, a post-rinse, a sealant and mixtures thereof.
 34. The method ofclaim 24 wherein said alloy product is selected from the groupconsisting of sheet product, plate product, an extrusion and a forging.35. The method of claim 34 wherein said sheet or plate product has atleast one bright exterior surface and is selected from the groupconsisting of: trailer plate, rail car skin, a tool box, vehicle runningboard and tread plate.
 36. The method of claim 34 wherein said sheetproduct is reflective lighting sheet.
 37. The method of claim 34 whereinsaid extrusion is selected from the group consisting of: a truckbedrail, hydraulic tubing, window framing, tub and shower framing, and agreenhouse structural support.
 38. The method of claim 34 wherein saidalloy product is a vehicle wheel.
 39. The method of claim 24 whichfurther includes: (b) rinsing material from the exposed exterior. 40.The method of claim 39 which further includes: (c) wiping said exposedexterior.
 41. The method of claim 24 which further includes: (b) wipingmaterial from the exposed exterior.
 42. The method of claim 24 whereinsaid 5000 Series aluminum alloy is selected from the group consistingof: 5083, 5086, 5454, 5657, 5182, and 5456 aluminum (AluminumAssociation designations).
 43. The method of claim 24 wherein said 6000Series aluminum alloy is selected from the group consisting of: 6061,6111 and 6022 aluminum (Aluminum Association designations).